The present invention relates to the art of recovering vapors, and more particularly vapors of solvents, monomers and hydrocarbons, from gas streams.
It is known to recover organic vapors by passing the gas stream through an adsorbent bed containing a substance such as activated carbon upon which the vapor is adsorbed. It is also known to desorb the organic vapor from the bed by maintaining a steady temperature in the bed and lowering pressure within the bed. Known systems are described in Skarstrom et al., Closed System Heatless Dryer, U.S. Pat. No. 3,225,518 (Dec. 28, 1965) and Kuri et al., Process for Concentrating or Liquefying a Specified Component of a Gaseous Mixture, U.S. Pat. No. 4,104,039 (Aug. 1, 1978), which are incorporated herein by reference.
Systems known in the art ordinarily use a plurality of adsorption beds. A feedstream containing organic vapor is passed through one bed under conditions at which adsorption will occur for a period of time short enough that the heat of adsorption remains substantially in the bed. Afterwards, the feedstream is redirected to a second bed. Presure is reduced in the first bed using a vacuum pump so that desorption occurs and a slight backpurge passing through the bed carries the desorbed organic vapor out of the bed. The desorbed vapor and backpurge gas pass through the pump to a codenser where at least some of the solvent is condensed and recovered. The remaining backpurge gas and desorbed vapor are recycled into the feedstream. The desorption step is halted in time to receive the feedstream back from the second bed so that the second bed can undergo desorption. Thereafter, the beds are alternately adsorbed and desorbed so that the system as a whole can be operated continuously.
In practice, the economic efficiency of such pressure swing adsorption systems is decreased by the limited choice of vacuum pumps which can be used in the system. The pump must draw enough of a vacuum to quickly and efficiently desorb the organic vapor which is adsorbed on the bed.
The least expensive and most efficient pumps for accomplishing the low pressures necessary are oil-sealed pumps. Oil-sealed pumps such as the rotary vane and rotary piston pumps cannot be used to recover oil-soluble compounds, such as 1,1,1-trichloroethane or styrene monomer, because the vapor dissolves in the oil as the desorbed vapor and backpurge gas pass through the pump. That thinning of the oil hurts the lubrication and seal within the pump and floods the pump.
Non-oil seal pumps capable of obtaining similar low pressures are ordinarily much more expensive than oil-sealed pumps.
An apparatus and a process are needed which permit the use of an oil-sealed pump in pressure swing adsorption to remove oil-soluble vapors from a gas stream.